Method and apparatus for real-time monitoring of an applied radiation dose

Abstract

The invention relates to a method for real-time monitoring of a radiation dose applied by an irradiation device in a target area using a magnetic resonance imaging scanner, as well as to a system comprising the magnetic resonance imaging scanner and the irradiation device. The irradiation device is configured to emit the radiation in a form modulated in intensity in the acoustic frequency range. The irradiation device radiates modulated radiation into the target area, and the magnetic resonance imaging scanner simultaneously acquires a magnetic resonance image of the target area using a sequence that is sensitive to an amplitude of a deflection of tissue in the target area.

Description

[0001] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identity are included.

[0002] The invention relates to a method for real-time monitoring of a radiation dose applied by an irradiation device in a target area using a magnetic resonance imaging scanner, as well as to a system comprising a magnetic resonance imaging scanner and an irradiation device. The irradiation device is configured to deliver the radiation in pulsed form. The magnetic resonance imaging scanner is configured to acquire a magnetic resonance image of the target area during irradiation.

[0003] In radiation therapy, the aim is to irradiate a target, such as a tumor, with a defined minimum dose while causing as little damage as possible to surrounding tissue. To this end, the precise location of the organs is recorded in preparation for radiation treatment, for example using computed tomography or magnetic resonance imaging. Based on this data, a radiation plan is created using model calculations, which determine the radiation directions and intensities for the treatment. Due to movements of individual organs and errors in the assumptions, e.g., regarding the individual properties of the individual tissue types, the actually applied radiation doses deviate from these model calculations.

[0004] From the article "Real-time, volumetric imaging of radiation dose delivery deep into the liver during cancer treatment | Nature Biotechnology" (https: / / www.nature.com / articles / s41587-022-01593-8) a method is known to record a dose distribution in the tissue using the sound waves generated by the radiation or its momentum and energy transfer.

[0005] The publication DE 10 2012 211581 A1 describes a method for elastography. The method comprises radiating a radiofrequency pulse for manipulating transverse magnetization in the specific region (30) and radiating at least one further radiofrequency pulse with spatially selective amplitude for generating shear waves in the specific region. The method comprises detecting a magnetic resonance signal from the specific region and determining a variable describing tissue elasticity in the specific region based on the magnetic resonance signal.

[0006] Magnetic resonance imaging scanners are imaging devices that, to create images of a subject, align the nuclear spins of the subject with a strong external magnetic field and then excite them to precess around this alignment using an alternating magnetic field. The precession, or return, of the spins from this excited state to a lower-energy state, in turn generates a response alternating magnetic field, which is received via antennas.

[0007] Using magnetic gradient fields, a spatial coding is imprinted on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated, providing a three-dimensional imaging representation of the object under examination.

[0008] It is an object of the present invention to make radiotherapy more reliable.

[0009] The object is achieved by a method according to the invention according to claim 1 and a system according to the invention according to claim 7.

[0010] The method according to the invention is intended for real-time monitoring of a radiation dose applied by an irradiation device in a target area using a magnetic resonance imaging scanner. Real-time monitoring is defined as monitoring that records the dose applied up to that point during a single irradiation session and makes it possible to change this dose or to modify the subsequent irradiation based on the recorded dose. Preferably, the dose is recorded for time periods of less than 10 s, 1 s, or 0.1 s. For example, the dose is determined for each of the radiation pulses explained below.

[0011] The irradiation device is designed to emit ionizing radiation for the treatment of tumors, for example. Ionizing radiation refers to photons with an energy greater than 1 keV, 10 keV, 100 keV, or 1 MeV. Such photons can be generated, for example, by an X-ray tube or an accelerator for charged particles, but also by a radioactive element. Ionizing particle beams with high-energy beta particles or electrons, protons, alpha particles, or even heavier ions are also conceivable. High-energy particles are considered to be particles with kinetic energy greater than 1 keV, 10 keV, 100 keV, or 1 MeV.

[0012] The irradiation device is designed to emit the emitted radiation in a form with modulated intensity. This means that the radiation output of the source changes over time, preferably by at least 10%, 30% or 50%. It is also conceivable for the intensity to change by up to 100%, in other words, for the radiation to be emitted in pulses. The frequency of the changes is in the acoustic frequency range, i.e. preferably in the range from 1 Hz to 100 kHz or 10 Hz to 10 kHz. In particular, the frequency is selected such that a pulse and / or energy transfer of the radiation can cause a mechanical movement or deflection in the tissue in the target area, which can then also be detected using magnetic resonance imaging. Shear or transverse oscillation as well as compression or longitudinal waves are conceivable.The deflection can be considered as a single deflection of volume elements of the tissue for a radiation pulse with subsequent return to the rest position, in the form of an exponentially damped oscillation or in an asymptotic manner. However, a periodic movement in the form of an oscillation is also conceivable.

[0013] The frequency of the modulation or the radiation pulses can also, in particular, correspond to a frequency of an image acquisition sequence of the magnetic resonance tomograph or be synchronized with its sequence, which, as explained below, is used to acquire a magnetic resonance image. For example, the irradiation can be pulsed and the pulse at a predetermined time interval from a reference point in the sequence, for example from an RF pulse and / or a gradient pulse. However, it is also conceivable that the predetermined time interval varies in a predetermined manner, for example in order to change a phase position during scanning. The synchronization can preferably be achieved via a signal connection between the magnetic resonance tomograph and the irradiation device, controlled by the magnetic resonance tomograph or the irradiation device, or in a mutual synchronization protocol.Special safety requirements of the irradiation device can be taken into account in order to exclude any risk to the patient due to irradiation lasting too long.

[0014] The magnetic resonance imaging scanner or the system comprising the magnetic resonance imaging scanner and the irradiation device is further configured to acquire a magnetic resonance image of the target area during irradiation. In other words, the magnetic resonance imaging scanner and the irradiation device are arranged relative to one another such that irradiation of a patient can occur while the patient is located in an image acquisition area of ​​the magnetic resonance imaging scanner, e.g., in the patient tunnel. The magnetic resonance imaging scanner must also have access for the radiation to the image acquisition area.

[0015] The method comprises the step of irradiating modulated radiation into the target area using the irradiation device.

[0016] Simultaneously, in one step of the method, a magnetic resonance image of the target area is acquired using the magnetic resonance imaging scanner. "Simultaneously" means that at least parts of the image acquisition sequence occur while the irradiation is taking place. The sequence is designed such that it can detect an amplitude of a deflection or oscillation of tissue in the target area at a frequency of the modulation of the irradiation modulation, i.e., the magnetic resonance signal is dependent on the amplitude of the deflection. Exemplary sequences for detecting the deflection include GRE-EPI, spin-echo, or a modified EPI sequence, as explained below with reference to the figures. The term "simultaneously" is preferably understood to mean that the irradiation or pulses are interleaved with the image acquisition sequence in a predetermined manner. Preferably, synchronization of irradiation and image acquisition occurs.This can be achieved, for example, via a signal connection between the irradiation device and the magnetic resonance imaging scanner. It is conceivable that one of the devices is the master, preferably the irradiation device for safety reasons, or the synchronization between the two takes place via a synchronization protocol in exchange or through another synchronization unit.

[0017] In a further step, a control system, either part of the magnetic resonance imaging system or separately, determines a dose distribution of the radiation in the target area from the acquired magnetic resonance image or as a function of the deflection or amplitude detected in the magnetic resonance image. The dose distribution can be presented, for example, in the form of a 2- or 3-dimensional map as a spatial distribution or spatially resolved information about the radiation dose.

[0018] The applied radiation dose is functionally related to the amplitude of the detected deflection, in that the deflection is caused by the effect of the radiation. A deflection due to a thermal effect, i.e., expansion due to heating, is conceivable. This expansion causes tissue to shift from a center of heating. A direct transfer of momentum from the radiation to the tissue is also conceivable, since high-energy photons also have momentum, and in particular particle beams consisting of particles with rest mass. The deflection can, for example, also depend on the radiation dose in a mathematically complex way, especially in the case of thermal effects. A radiation dose can be determined from this using calibration or mathematical modeling.Since the deflection depends, among other things, on the power and / or momentum of the beam, the dose is calculated by integrating the deflection over time. This integration can be determined mathematically or, preferably, directly from the sequence used for acquisition, as explained in more detail below with the figures. Thus, a spatial distribution or map of the applied radiation dose can be determined from the acquired amplitude distribution, for example, by controlling the magnetic resonance imaging system or another control system.

[0019] The method according to the invention advantageously uses the radiation-induced deflection and its detection by means of a magnetic resonance sequence to determine a radiation dose applied to a patient in real time and in a location-related manner.

[0020] Further advantageous embodiments are specified in the subclaims.

[0021] In a preferred embodiment of the method according to the invention, the method further comprises the step of acquiring a reference magnetic resonance image of the target area using the magnetic resonance imaging scanner without simultaneous irradiation by the irradiation device. Preferably, the acquisition of the reference magnetic resonance image is carried out using a sequence that is identical except for the irradiation.

[0022] The reference magnetic resonance image is then used in the dose rate determination step. The effect of radiation-induced deflection and its consequences are small, even with optimized sequences, and can be overlaid by other effects such as B0 or ​​B1 inhomogeneities. These disturbing effects can be advantageously reduced by subtracting image data with and without radiation.

[0023] In one conceivable embodiment of the method according to the invention, a sequence with motion encoding gradients (MEG) is used in the step of acquiring the magnetic resonance image and the step of acquiring the reference magnetic resonance image. A motion encoding gradient is designed to change the phase and / or amplitude of the precession of nuclear spins of a slice or volume element, so that a spatial displacement of the volume element or slice can be detected by measuring the magnetic resonance signals. Preferably, a gradient along the deflection is used for this purpose, so that nuclear spins subject to deflection are at least temporarily exposed to a different magnetic field B0 plus the gradient strength times the amplitude of the deflection, and are thus subjected to a phase change relative to nuclear spins that are not deflected (in the direction of the gradient).Sequences with such gradients are known, for example, from the elastography publications mentioned above. Therefore, the motion encoding gradient is preferably essentially parallel to the direction of the deflection. It is also conceivable to use image acquisition with gradients in three dimensions to capture spatial thermal expansion. Image acquisition with the applied MEG(s) occurs while the beam is incident on the target area.

[0024] Magnetic resonance sequences or spin labeling sequences such as those used for flow MRI are also conceivable for diffusion measurements.

[0025] In the step of acquiring a magnetic resonance image and / or the step of acquiring a reference magnetic resonance image, the acquisition with inverted polarity of a motion encoding gradient takes place.

[0026] Advantageously, a pulse with inverted polarity when acquired as a so-called bipolar or "balanced" motion encoding gradient ensures that artifacts, such as those caused by the susceptibility of metals or susceptibility jumps between different tissue types or tissue and air, are canceled out due to the reversed signs and only the motion-related effects remain.

[0027] In one possible embodiment of the method according to the invention, the method further comprises the step of acquiring a temperature magnetic resonance image of the target area by means of an image acquisition by the magnetic resonance imaging scanner using a temperature-sensitive sequence. This can be determined by evaluating the temperature-dependent change in the Larmor frequency or a temperature-dependent change in the T1 relaxation time. In a further step, a temperature map of the target area is determined from the temperature magnetic resonance image.

[0028] Advantageously, a radiation dose determined by movement can be verified by its thermal effect. Furthermore, the effect of radiation on the tumor is temperature-dependent, allowing for a better assessment of the radiation dose and the radiation dose to be optimized or reduced while maintaining the same therapeutic success.

[0029] In one conceivable embodiment of the method according to the invention, a position of a target tissue, for example a tumor, but also of a neighboring organ to be protected, is determined from one of the acquired magnetic resonance images, and further irradiation is carried out depending on the determined position. Due to natural movement of the body, for example breathing, heartbeat or peristalsis, but also due to voluntary movements, the position of a tumor can change during the course of treatment and in particular between different treatment appointments due to treatment-induced shrinkage, even with careful external marking and positioning, so that on the one hand the irradiation does not hit the target, e.g. the tumor, or does not hit it completely, and instead damages neighboring important organs.

[0030] Because the magnetic resonance imaging scanner simultaneously acquires a spin distribution for a "normal" image when acquiring images for dose determination, e.g. when an evaluation is carried out without difference formation, this quasi real-time imaging can be used in an advantageous manner and the radiation can be adjusted in an advantageous manner, be it by reorienting the radiation device, the patient or even just by an interruption, e.g. during breathing or a heartbeat, until the tumor and / or neighboring organs have returned to the predetermined position of the radiation planning.

[0031] In one possible embodiment of the method according to the invention, the method further comprises the step of achieving a predetermined heating in the target area using a radio-frequency signal from the magnetic resonance imaging scanner. The radio-frequency pulses required to excite the nuclear spins always lead to heating of the body tissue due to the absorbed radio-frequency energy. By appropriately designing the radio-frequency pulses, for example, by emission via multiple antennas for spatial modification, or by additional radio-frequency pulses, the heating can be influenced to achieve a predetermined temperature increase in a predetermined target area.

[0032] Increasing the temperature can advantageously optimize the effect of radiation on the tissue. Furthermore, thermometry using magnetic resonance imaging, as already described, allows this effect to be adjusted and monitored more precisely.

[0033] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which are explained in more detail in connection with the drawings.

[0034] They show: Fig. 1 is a schematic representation of a device according to the invention; Fig. 2 is a schematic flow chart of a method according to the invention; Fig. 3 is a schematic representation of the radiation effect on tissue of a patient; Fig. 4 is a temporal relationship between radiation and deflection; Fig. 5 is an exemplary schematic temporal course of a GRE-EPI sequence for use in the invention; Fig. 6 is an exemplary schematic temporal course of a modified SE sequence for use in the invention; Fig. 7 is an exemplary schematic temporal course of a modified EPI sequence for use in the invention.

[0035] Fig. 1 shows a schematic representation of an exemplary embodiment of a device according to the invention for real-time monitoring of an applied radiation dose.

[0036] The device of Fig. 1essentially comprises a magnetic resonance imaging device 9 and an irradiation device 200, the control units 20, 201 of which are connected to one another via a signal connection 203 in order to carry out the real-time monitoring of the method according to the invention.

[0037] The magnet unit 10 of the magnetic resonance imaging system 9 has a field magnet 11 that generates a static magnetic field B0 for aligning nuclear spins of samples or patients 100 in a recording area. The magnet unit 10 in Fig. 1is shown by way of example as a horizontal split magnet, i.e. the magnet unit 10 is divided or subdivided into two parts so that the irradiation device 200 has free access for irradiation to the patient 100. Other embodiments of the magnet unit 10 are also conceivable which allow access for irradiation and, at the same time, image acquisition in the target area, for example, vertical split magnet systems or one-piece magnet units 10 with corresponding openings. In principle, radiation sources are also conceivable as the irradiation device 200 within the magnet unit 1, provided that their radiation intensity can be modulated according to the invention and they are compatible with image acquisition.

[0038] The acquisition area of ​​the magnetic resonance imaging scanner is arranged in a patient tunnel 16, which extends in a longitudinal direction 2 through the magnet unit 10. As already explained above, the patient tunnel 16 is preferably at least partially interrupted, so that the patient is accessible for irradiation by the irradiation device 200. The acquisition area is located in a region that also includes the target area of ​​the irradiation in the patient 100.

[0039] The patient 100 can be moved into the recording area using the patient couch 30 and the traversing unit 36 ​​of the patient couch 30. Typically, the field magnet 11 is a superconducting magnet capable of generating magnetic fields with a magnetic flux density of up to 3T, and even higher in the latest devices. However, permanent magnets or electromagnets with normally conducting coils can also be used for lower field strengths.

[0040] Furthermore, the magnet unit 10 has gradient coils 12, which are designed to superimpose variable magnetic fields in three spatial directions on the magnetic field B0 for spatial differentiation of the acquired imaging regions in the examination volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate fields with mutually orthogonal gradients in the examination volume.

[0041] The magnet unit 10 also has a body coil 14, which is designed to transmit a high-frequency signal supplied via a signal line into the recording area and to receive resonance signals emitted by the patient 100 and transmit them via a signal line. Preferably, however, the body coil 14 for transmitting and / or receiving the high-frequency signal is replaced by local coils arranged in the patient tunnel 16 close to the patient 100. However, it is also conceivable that the local coil is designed for transmitting and receiving, and therefore a body coil 14 can be omitted.

[0042] A control unit 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14 and evaluates the received signals. A magnetic resonance imaging controller 23 coordinates the subunits.

[0043] The control unit 20 has a gradient control 21 which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the examination volume in a time-coordinated manner.

[0044] Furthermore, the control unit 20 has a radio-frequency unit 22, which is designed to generate a radio-frequency pulse with a predetermined temporal profile, amplitude, and spectral power distribution for exciting a magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The individual units are interconnected via a signal bus.

[0045] The radio-frequency signal generated by radio-frequency unit 22 is fed to the body coil 14 via a signal connection and transmitted into the body of the patient 100 to stimulate the nuclear spins there. However, it is also conceivable to transmit the radio-frequency signal via one or more local coils.

[0046] The local coil then preferably receives a magnetic resonance signal from the body of the patient 100, because due to the short distance, the signal-to-noise ratio (SNR) of the local coil is better than when received by the body coil 14. The MR signal received by the local coil is processed in the local coil and forwarded to the radio frequency unit 22 of the magnetic resonance tomograph 1 for evaluation and image acquisition.

[0047] The irradiation device 200 is arranged in a position relative to the magnetic resonance imaging scanner 9 such that a beam can reach a target area in a recording area of ​​the magnetic resonance imaging scanner 9. In the case of a split magnet, it is also conceivable for the irradiation device 200 to be moved along a path around the target area in order to reduce damage to tissue around the target, e.g., a tumor.

[0048] The irradiation device 200 is capable of modulating the emitted radiation in a predetermined manner with an acoustic frequency, for example, in the frequency range from 1 Hz to 100 kHz or 10 Hz to 10 kHz. An X-ray source or an accelerator is conceivable, in which the intensity can be varied by a current from the ion or electron source. Radiation sources with an isotope would also be conceivable, provided that a controllable shutter can achieve modulation or interruption at the aforementioned frequencies in a predetermined manner.

[0049] In Fig. 2 A flow diagram of an embodiment of the method according to the invention is shown schematically.

[0050] In a step S20, intensity-modulated radiation is applied to the target area by means of the irradiation device 200. The modulation occurs at an acoustic frequency, preferably in a range of 1 Hz to 100 kHz or 10 Hz to 10 kHz. The modulation has a predetermined time or phase relationship to an image acquisition of the magnetic resonance imaging scanner 9; in other words, the modulation is synchronized with the image acquisition. Synchronization occurs via a signal connection 203 between a controller 23 of the magnetic resonance imaging scanner 9 and a control unit 201 of the irradiation device 200.

[0051] In step S30, the magnetic resonance imaging system 9 acquires a magnetic resonance image of the target area during the irradiation. "During" here means that the magnetic resonance image encompasses at least the period of irradiation, so that the magnetic resonance image can capture the entire irradiated radiation dose. Preferably, irradiation periods are synchronized with, or occur simultaneously with, image acquisition periods that are sensitive to a deflection of the tissue or the nuclear spins contained therein.

[0052] In Fig. 3It is schematically shown how the incident radiation leads to a deflection in the tissue. The incident radiation, be it X-rays or gamma quanta or even high-energy particles, introduces energy into the tissue as well as a momentum P. If the radiation is scattered or absorbed by the atoms in the tissue, the energy is largely converted into heat, which leads to thermal expansion, which tissues as in Fig. 3 As indicated by the concentric circles, the beam can be expanded around a center with a maximum energy input. This expansion results in radial movement from this center in all directions. When determining the radiation dose from the measured deflection, these geometric relationships must be taken into account, especially if the thermal effect predominates.

[0053] However, the momentum P is also transferred, in whole or in part, to the atoms, and a force is exerted on the atoms, which in turn can cause movement along the beam direction, as indicated by the arrows labeled Δy. Since the atoms are elastically bound in the tissue, the force can also lead to deflection. Which effect predominates depends, among other things, on the type of radiation and its energy. Ion radiation, for example, leads to a higher momentum transfer due to the high rest mass of the particles.

[0054] For the non-oscillatory deflection, the deflection can be modeled as follows: a r → t = { a max ⋅ 1 − e − t − t 1 τ rise , t 1 ≤ t ≤ t 2 a pk ⋅ e − t − t OFF τ decay , t > t 2

[0055] The time constants τ rise and τ decay affect the elastic properties of the tissue. In muscle tissue, the rise time constant τ rise for example 3.2 ms and the decay time constant τ decay5.5 ms. In liver tissue, the rise time is 6.2 ms and the fall time is 7.9 ms. The parameter a max is the quasi-static maximum of the deflection for continuous irradiation.

[0056] With the parameter a pk = a max ⋅ 1 − e − t 2 − t 1 τ rise indicates the maximum deflection of the tissue that is reached at the end of a radiation pulse.

[0057] In Fig. 4 The temporal relationship between the incident radiation and the resulting tissue deflection is shown schematically. For simplicity, the modulated radiation is represented as a rectangular pulse, which is defined by switching the radiation source on at a predetermined time t1 and off at a predetermined time t2.

[0058] The thermal effect, in its temporal progression, is primarily determined by the heat capacity of the tissue and its thermal conductivity. The temperature, and thus the thermal expansion and deflection of a volume element, increases with increasing temperature due to the energy supplied by the radiation. At the same time, the heat dissipation increases with the temperature difference. The deflection initially increases steeply and then asymptotically transitions to an equilibrium state unless the beam is interrupted beforehand. After the interruption, the temperature decreases again due to heat diffusion, asymptotically decreasing to body temperature.

[0059] A force caused by a pulse of radiation, however, occurs instantaneously with the onset of the radiation and is approximately constant over time. The tissue essentially follows the model of a damped pendulum. The damping is caused by the viscosity of the tissue, and the restoring force is caused by the elasticity of the tissue for transverse movement. Fig. 4 The case is shown where the viscosity of the tissue leads to an asymptotic oscillation. When the radiation is switched on, the deflection asymptotically approaches a maximum equilibrium value a max , and drops asymptotically to 0 after the beam is switched off.

[0060] In this respect, the exemplary course of the deflection in Fig. 4 qualitatively comparable for both cases and depends essentially on the properties of the beam, the tissue and the beam duration.

[0061] Different examples of suitable sequences and time sequences are shown in the following figures Fig. 5 to Fig. 7 specified.

[0062] In step S50, a dose distribution of the radiation in the target area is determined as a function of the detected amplitude of the deflection. In the simplest case, the deflection is proportional to the radiation intensity or power. The radiation dose is therefore proportional to an integral of the deflection over time. As explained below for the sequences, the integral can already be realized by the sequence. Alternatively, it is conceivable to perform the integral numerically. A proportional factor can be achieved, for example, by a calibration sequence on a phantom with a defined radiation dose.

[0063] Preferably, in a step S40, a reference magnetic resonance image of the target area is acquired with the magnetic resonance tomograph 9 without irradiation by the irradiation device 200. In this way, a reference image is acquired that reflects influences caused by the patient 100 and inhomogeneities in the image acquisition of the magnetic resonance tomograph 9. For example, by forming the difference in step S50 to determine the dose distribution between the magnetic resonance image with irradiation and the reference magnetic resonance image without irradiation, these inhomogeneities can be advantageously eliminated.

[0064] To detect the deflection in the magnetic resonance image, so-called motion encoding gradients (MEG) are used, as explained in more detail below with regard to the exemplary sequences. These gradients are used to generate a phase deviation of the magnetic resonance signals from the tissue deflection. However, susceptibility jumps in the tissue can cause similar phase changes from the motion encoding gradients. Since the susceptibility effects change their sign with the polarity of the gradients, in contrast to those caused by the deflection, in a preferred embodiment, steps S30 of acquiring the magnetic resonance image and S40 of acquiring the reference magnetic resonance image are each repeated with the inverted polarity of a phase encoding gradient.Subsequently, in step S50, both the magnetic resonance images and the reference magnetic resonance images with both polarities of the motion encoding gradients are used to compensate for the susceptibility effects when determining the dose distribution.

[0065] It is also conceivable that, in step S10, a predetermined heating occurs in the target area using a radio-frequency signal from the magnetic resonance imaging scanner. For example, a body coil with a plurality of transmission channels or a local transmission array can achieve a predetermined field distribution, which achieves a predetermined heating in the target area by absorbing the radio-frequency waves. However, it is also conceivable that the predetermined heating occurs during or after irradiation with the radiation pulse.

[0066] It is also conceivable that in a step S60 a temperature magnetic resonance image of the target area is acquired by means of an image acquisition with a temperature-sensitive sequence and in a further step S70 a temperature map of the target area is determined from the temperature magnetic resonance image.

[0067] Below are some possible sequences that have a sensitivity for detecting a deflection.

[0068] Fig. 5 shows, as an example, the temporal course of signals of a GRE-EPI sequence in temporal correlation with the irradiation and its detectable effect on the tissue.

[0069] The time axis runs from left to right. Vertical lines are inserted to highlight simultaneous events. The sequence is not shown in its entirety, as indicated by the break lines on the right, but only a representative beginning of the sequence.

[0070] RF refers to the radiofrequency signal that is radiated into the patient 100 to excite the nuclear spins. In the GRE-EPI sequence, a radiofrequency pulse is first applied to excite a predetermined slice, simultaneously with a slice selection gradient.

[0071] In the Fig. 5 to 7 Below, three gradient signals are plotted: gME, gR, and gP. X, y, and z are not used as labels here, since the axes are interchangeable, depending on the direction in which a deflection is to be measured.

[0072] Here, gME denotes the gradient used for motion encoding. For the GRE sequence, motion encoding is performed with the same gradient used for slice selection during nuclear spin excitation. gR denotes the gradient used for readout, and gP the gradient used for phase encoding. The gradients of the vector fields generated by the gradient coils preferably span a Cartesian coordinate system, with a 2D slice being scanned in the plane spanned by gR and gP. The strength of the gradients is plotted in arbitrary units, with positive and negative polarities relative to the rest line indicated.

[0073] RT indicates the temporal variation of the radiation intensity of the irradiation device 200 in arbitrary units. For simplicity, a pulse is assumed here due to switching the beam on and off.

[0074] A indicates the amplitude of the deflection in the tissue. Fig. 4 As already explained, the deflection begins with the radiation and falls back to the rest position after the radiation has ended.

[0075] In the GRE-EPI sequence, encoding by gME occurs in a section without radiation or deflection induced thereby with one polarity of the gradient, and in a section with radiation or deflection with an inverse polarity of the gradient. By subtracting the signals, non-deflection-related effects can be reduced, thus increasing the sensitivity to the deflection. By repeating the sequence shown, scanning occurs with inverted polarities, as indicated by the curves labeled phi+ and phi-. The deflection can thus be recorded with different gradient field directions. Such so-called balanced gradient pulses allow the effects of deflection to be distinguished from signals induced by susceptibility jumps, since the direction or polarity of the signals induced by susceptibility jumps with the reversed polarity also changes direction or polarity.In this way, susceptibility effects can also be reduced by difference formation.

[0076] The following relationship results for a phase change of nuclear spins: Δ φ r → t = γ ∫ 0 t MEG → r → τ ⋅ u → r → τ dτ

[0077] With φ for the phase, r for the location of a nuclear spin, MEG ( r,τ ) the gradient gME as a function of location and time, u ( r,τ ) as a vector for the deflection of a nuclear pin depending on the resting position and time as well as γ as a gyromagnetic factor.

[0078] For bipolar and so-called "balanced" MEG and without deflection in the tissue during MEG, the term is zero.

[0079] The sequence is preferably repeated 4 times, the first time with positive MEG pulses superimposed on the slice selection pulses (in the Fig. 5 in gME denoted by phi+), a second time with positive MEG, but without radiation pulses RT, the third time with negative MEG pulses (in the Fig. 5 (in gME, denoted by phi-) and the fourth time again without radiation pulses RT. The four cumulative phase shift maps acquired with these four magnetic resonance sequences are: { ∅ + x y ∅ 0 + x y ∅ − x y ∅ 0 − x y mit positivem MEG und ohne RT pulse mit negativem MEG und ohne RT Pulse

[0080] The phase background maps, acquired without a radiation pulse, provide the calibration data to compensate for the effects of eddy currents, since the EPI sequence is very sensitive to stray fields from surrounding conductors induced by the gradient fields.

[0081] This gives the deflection map of the tissue: a x y = ∅ + x y − ∅ 0 + x y − ∅ − x y + ∅ 0 − x y γ ⋅ G MEG ⋅ 2 δ

[0082] Where G MEG the amplitude and 2 δ is the duration of the MEG pulses This results in a temperature map with Δ T x y = ∅ + x y − ∅ 0 + x y + ∅ − x y ∅ 0 − x y γ ⋅ TE ⋅ B 0 ⋅ 2 α where α = -0.0094 ppm / °C is the temperature coefficient for the proton resonance frequency shift (PRFS), TE is the echo time and B0 is the strength of the static magnetic field B0 in Tesla.

[0083] Fig. 6 shows an exemplary standard magnetic resonance sequence SE modified for sensitivity to tissue displacement.

[0084] The RF radiofrequency pulses comprise a 90° pulse to tilt the magnetization into the transverse plane, followed by a 180° refocusing pulse, both performed at the Larmor frequency or resonance frequency (e.g., 64MHz for a 1.5T magnetic resonance imaging scanner).

[0085] Additionally, gradients that increase the deflection sensitivity are added to the standard sequence in the slice selection direction to encode the radiation-induced deflection. These are displayed on gME as sinusoidal pulses.

[0086] A second radiation pulse is delivered synchronously with the second MSG period to cause synchronous tissue displacement and minimize the radiation pulse duration. The intensity and duration of the radiation pulse RT are selected to cause sufficient tissue displacement for acquisition by the sequence, while remaining low enough to minimize tissue damage. The MR signals with the accumulated phase shift are then acquired during the readout window.

[0087] In Fig. 7Another modified EPI sequence is shown, which takes into account the influence of other non-radiation-induced tissue displacements, for example, due to respiration or heartbeat. The rapid succession of three EPI sequences captures the phase accumulation in the reconstructed magnetic resonance images with sine- and cosine-modulated MEG pulses and a third magnetic resonance image with MEG pulses but without radiation pulses. This final sequence measures the accumulated phase background caused by other types of slower motion. Advantageously, the sine- and cosine-modulated gradient pulses reduce both eddy current effects and the acoustic noise radiated by the gradient pulses.

[0088] Within the scope of the invention, further variations of sequences are also conceivable that provide sensitivity to tissue deflections. In particular, other sequences already used in elastography are conceivable. Sequences used in image acquisition with contrast agents are also conceivable.

[0089] Furthermore, sequences are conceivable that are used for diffusion measurements, since these are not limited to a predetermined linear direction of movement, but also record a spherically symmetric movement due to thermal expansion.

[0090] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

1. A method for real-time monitoring of a radiation dose applied by an irradiation device in a target area by means of a magnetic resonance tomograph, wherein the irradiation device is designed to emit the radiation in a form modulated in intensity in the acoustic frequency range, wherein the magnetic resonance tomograph is designed to acquire a magnetic resonance image of the target area during the irradiation, the method comprising the steps of: (S20) irradiating modulated radiation into the target area by means of the irradiation device; (S30) acquiring a magnetic resonance image of the target area during the irradiation with the magnetic resonance tomograph, wherein the magnetic resonance tomograph applies a sequence designed to acquire an amplitude of a deflection of tissue in the target area; (S50) determining a dose distribution of the radiation in the target area as a function of the acquired amplitude of the deflection.

2. The method according to claim 1, wherein the method further comprises the step (S40) of acquiring a reference magnetic resonance image of the target area without irradiation, and in the step of determining the dose distribution, the dose distribution is determined as a function of the magnetic resonance image and the reference magnetic resonance image.

3. The method according to claim 1 or 2, wherein in the step (S30) of acquiring the magnetic resonance image and the step (S40) of acquiring the reference magnetic resonance image, a sequence with motion encoding gradients is used and the step of acquiring a magnetic resonance image and / or the step of acquiring a reference magnetic resonance image is carried out with inverted polarity of a phase encoding gradient.

4. The method according to any one of the preceding claims, wherein the method further comprises the step (S60) of acquiring a temperature magnetic resonance image of the target area by means of image acquisition with a temperature-sensitive sequence and, in a further step (S70), a temperature map of the target area is determined from the temperature magnetic resonance image.

5. Method according to one of the preceding claims, wherein a position of a target tissue is determined from one of the acquired magnetic resonance images and further irradiation is carried out depending on the determined position.

6. The method according to any one of the preceding claims, wherein the method further comprises the step (S10) of achieving a predetermined heating in the target area by means of a high-frequency signal of the magnetic resonance imaging device.

7. A system for real-time monitoring of a radiation dose applied by an irradiation device in a target area, the system comprising the irradiation device and a magnetic resonance imaging scanner, the irradiation device being configured to emit the radiation in a form modulated in intensity in the acoustic frequency range, synchronized with an image acquisition of the magnetic resonance imaging scanner, and to irradiate the radiation into the target area; the magnetic resonance imaging scanner being configured to acquire a magnetic resonance image of the target area during irradiation, the magnetic resonance imaging scanner applying a sequence configured to acquire an amplitude of a deflection of tissue in the target area; the system being configured to determine a dose distribution of the radiation in the target area as a function of the acquired amplitude of the deflection of the tissue.

8. A computer program product comprising a program and being directly loadable into a memory of a programmable controller (23) of a system according to claim 7, with program means for carrying out all the steps of the method according to any one of claims 1 to 6 when the program is executed in the controller (23).

9. Electronically readable data carrier with electronically readable control information stored thereon, which is designed such that it carries out the method according to one of claims 1 to 6 when the data carrier is used in a controller (23) of a system according to claim 7.

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